Lithium ion batteries have occupied a prominent position in conventional power sources due to the capability to provide high energy density and good cycling performance. However, conventional commercial grade lithium ion batteries have been subject to safety issues resulting from the use of flammable organic solvents as electrolyte components. Moreover, lithium is toxic and of high cost and ongoing research efforts in power source development have targeted batteries of improved safety and lower cost.
Further, the use of electricity generated from environmentally clean and renewable sources, such as water, wind, or sunlight requires high-power and high-energy secondary batteries to efficiently store and redistribute the electrical energy Ideally, such batteries will be constructed using abundant, low-cost materials according to sustainable processes. Conventional secondary (rechargeable) batteries include lithium, lithium ion (LIB), sodium ion, nickel cadmium (Ni—Cd), lead-acid, magnesium, calcium and aluminum batteries. Most of the current batteries, such as for example lithium ion batteries, utilize univalent ions (i.e. H+, Li+, Na+ or K+) as the medium to store energy.
However, multivalent ions are increasingly of interest for use in battery chemistry to establish a “next generation” of batteries to the lithium battery. Performance of selected multivalent ions in aqueous electrolyte media has the potential to meet and exceed the performance of lithium batteries with greater safety and lower cost. The need for such next generation of batteries is exacerbated because of the rapid development and commercialization of electrified vehicles such as electric vehicles (EV), plug-in hybrid vehicles (PHV) and hybrid vehicles (HV).
Multivalent ions, including Ni2+, Zn2+, Mg2+, Ca2+, Ba2+, and La3+ are under investigation for utility in the next generation battery. However, each of these ions may require specific combinations of anode composition and structure, cathode composition and structure and electrolyte composition. In general it has not been possible to simply substitute multivalent ions in present lithium ion battery systems to successfully derive new batteries of commercial utility.
Of the potential multivalent ions listed above. Zinc is a strong candidate due to an abundant supply, thus, low cost and environmental friendliness. The redox potential of Zn/Zn2+ is −0.762 V vs. SHE (or 2.278 vs. Li/Li+), and thus, zinc provides adequately low potential as an anode material when coupling with high voltage cathodes.
Moreover, since the current Li-ion secondary battery uses a flammable non-aqueous electrolyte, it is necessary in present vehicles to install external battery control circuits and internal safety components. Such systems incur added weight, bulk and cost to the vehicle. On the other hand, batteries using an aqueous (water-based) electrolyte, such as Ni−MH battery, are much safer because of the inflammable property of the electrolyte, however, the energy density of aqueous electrolyte batteries is quite low due to their low voltage profile (<1.6 V) caused by the limitation of the electrochemical stability of the aqueous electrolyte.
Zinc has a high theoretical capacity (820 mAhg−1) as anode. An adaptable negative potential (−0.762 V vs. SHE), enables zinc to be constructed with other cathodes in aqueous electrolytes. Several rechargeable zinc batteries, including nickel//zinc battery, zinc//air battery and Zn//Na0.95MnO2 and Zn//LiMn2O4 have been investigated in aqueous systems.
However, the selection of cathodes to combine with a zinc based anode may be constrained by the limitation of the cathode potential vs. Zn2+/Zn. Too low cathode potential also reduces the power density of cathodes, while too high potential may destroy the aqueous electrolyte, due to decomposition of water. To be utilized as an aqueous cathode the cathode active material must have aqueous stability and adaptable charge/discharge potential plateaus compared with that of decomposition of water. Cathodic 4 V active materials such as LiMn2O4, LiCoO2 or LiN1/3Co1/3Mn1/3O2 offer high performance opportunity, however their potential plateaus in aqueous media are close to the decomposition potential of water. Therefore, cathodes with potential plateaus below 4 V have been investigated in aqueous electrolyte batteries to date.
Such low V cathode materials studied include the olivine structured LiFePO4 and NASICON-type M3V2(PO4)3 (M=Li, Na) which were paired with Zn metal as anode in the form of hybrid Zn—Li batteries. However, the achieved discharge voltage obtained is insufficient due to the limited potential of the cathodes.
Therefore, there is a need to identify materials and construction capable of utilizing zinc chemistry as the anode component in combination with an aqueous electrolyte to construct a secondary (rechargeable) battery of high discharge voltage, good cycle discharge stability and high safety performance.